An M_S4.9 earthquake occurred at 08:17 on the December 27, 2016 in Rongchang District, Chongqing, and the epicenter is located in the north central section of Huayingshan basement fault system on the eastern margin of Sichuan Basin. The seismicity shown in the historical earthquake catalogue was originally very weak in this area. Since the late 1980s, due to the impact of waste water reinjection in the natural gas field, earthquakes of magnitude ≥4.0 occurred frequently and 14 earthquakes with M_S≥4.0 have occurred, the largest of which was Rongchang M_S5.0 earthquake in 1997. In this paper, the fine three-dimensional P-wave velocity structures and relocation results of seismic events in Rongchang and its surrounding areas are inversed by double difference tomography method, based on the P-wave and S-wave arrival time data of 1786 seismic events recorded by Chongqing regional fixed network, mobile network and Zigong local network from January 2008 to June 2020.
The results show that: 1)The distribution of high-velocity and low-velocity zones within 4km depth is significantly different from that below 7~13km depth. The P-wave high-velocity zone at 4km depth is mainly distributed in Renyi-Rongchang area, where there are four water injection wells, a major concentration area of continuous water injection in Rongchang since 2008. The range of Renyi-Rongchang high velocity zone significantly gets narrowed at the 7km depth and is obviously different from that at the shallow surface. The velocity structures on the east and west sides of Huayingshan basement fault vary greatly from 7 to 13km. The P-wave velocity structures of different sections across Huayingshan basement fault all indicate that the depth of the interface between the sedimentary cover and crystalline basement is 12km in Rongchang area, which is basically consistent with the previous research results and the characteristics of seismic reflection profiles in Rongchang area. The inversed velocity structures well mirror the shape of Luoguanshan fold, and further confirm the reliability of our results. 2)The lateral difference of P-wave velocity structure in the shallow layer of Rongchang area varies greatly. There is a high-velocity zone near the Luo2^# water injection well at the axis of Luoguanshan anticline and the depth distribution is 3~7km. The hidden fault in the north wing of Luoguanshan anticline with buried depth of 1.7km is developed near well Luo2^#, and the high velocity zone is distributed along the dip of the hidden fault, which may indicate that the hidden fault may be the main channel for wastewater infiltration. The depth of wastewater infiltration is up to 7km, resulting in a large velocity difference between the two sides of the fault. The M_S4.9 earthquake on December 27, 2016 and the M_S4.0 earthquake on December 28, 2016 are just distributed in the velocity transition zone. Obvious high-velocity body was not found below 3km in Luo4^# water injection well, which may be related to the cessation of water injection in Luo4^# well in February 2001. 3)The results of seismic relocation indicate that earthquakes are mainly distributed in the axis of the strongly deformed Luoguanshan anticline, showing obvious stripe distribution in NE direction, and the focal dominant depth is 0~6km. Based on the focal mechanism solution and the regional seismotectonic environment, it is believed that the seismogenic fault of earthquakes above M_S4.0 on the south side of Guangshun transverse fault should be the hidden fault on the south wing of Luoguanshan, while the seismicity on the north side of Guangshun transverse fault may be related to the hidden fault on the north wing of Luoguanshan.

The Wulong M_S5.0 earthquake on 23 November 2017, located in the Wolong sap between Wenfu, Furong and Mawu faults, is the biggest instrumentally recorded earthquake in the southeastern Chongqing. It occurred unexpectedly in a weak earthquake background with no knowledge of dramatically active faults. The complete earthquake sequences offered a significant source information example for focal mechanism solution, seismotectonics and seismogenic mechanism, which is helpful for the estimation of potential seismic sources and level of the future seismic risk in the region. In this study, we firstly calculated the focal mechanism solutions of the main shock using CAP waveform inversion method and then relocated the main shock and aftershocks by the method of double-difference algorithm. Secondly, we determined the seismogenic fault responsible for the M_S5.0 Wulong earthquake based on these calculated results. Finally, we explored the seismogenic mechanism of the Wulong earthquake and future potential seismic risk level of the region.
The results show the parameters of the focal mechanism solution, which are:strike24°, dip 16°, and rake -108° for the nodal plane Ⅰ, and strike223°, dip 75°, and rake -85° for the nodal plane Ⅱ. The calculations are supported by the results of different agencies and other methods. Additionally, the relocated results show that the Wulong M_S5.0 earthquake sequence is within a rectangular strip with 4.7km in length and 2.4km in width, which is approximately consistent with the scales by empirical relationship of Wells and Coppersmith(1994). Most of the relocated aftershocks are distributed in the southwest of the mainshock. The NW-SE cross sections show that the predominant focal depth is 5~8km. The earthquake sequences suggest the occurrence features of the fault that dips northwest with dip angle of 63° by the least square method, which is largely consistent with nodal planeⅡof the focal mechanism solution. Coincidentally, the field outcrop survey results show that the Wenfu Fault is a normal fault striking southwest and dipping 60°~73° by previous studies. According to the above data, we infer that the Wenfu Fault is the seismogenic structure responsible for Wulong M_S5.0 earthquake.
We also propose two preliminary genetic mechanisms of "local stress adjustment" and "fluid activation effect". The "local stress adjustment" model is that several strong earthquakes in Sichuan, such as M8.0 Wenchuan earthquake, M7.0 Luzhou earthquake and M7.0 Jiuzhaigou earthquake, have changed the stress regime of the eastern margin of the Sichuan Basin by stress transference. Within the changed stress regime, a minor local stress adjustment has the possibility of making a notable earthquake event. In contract, the "fluid activation effect" model is mainly supported by the three evidences as follows:1)the maximum principle stress axial azimuth is against the regional stress field, which reflects NWW-SEE direction thrusting type; 2)the Wujiang River crosscuts the pre-existing Wenfu normal fault and offers the fluid source; and 3)fractures along the Wenfu Fault formed by karst dissolution offer the important fluid flow channels.

Dazu Well is located in Shiwan Town of Dazu County,Chongqing City,the well is 108.7m deep. The observed aquifer layer is weather-fractured phreatic aquifer,of which the rock stratum is mudstone of Middle Jurassic Shaximiao Formation(J_2S)interbedded by sand. Years of observations show that whenever the nearby well,which is 14.6m away,was pumped,the water level of Dazu well rose instead of drop. This kind of anomalous response has been rarely reported both at home and abroad. By analyzing the observation data,investigating the local geological and hydrogeological conditions,field observation and theoretic analysis,etc. ,we conclude that the anomalous response of Dazu well water level to the pumping of nearby well is attributed to the special water level regime of the observation well caused by hydrodynamic pressure due to the larger flow velocity in the outer boundary of pumping-depression cone,under a specific hydrogeological condition.

The Kunlunshan M_S 8.1 earthquake of Nov.14,2001 is the biggest seismic event in China's continent in the past five decades. For this event,some researchers have developed an analysis method of brightness temperature difference inside and outside the Eastern Kunlun Fault,and have drawn an important conclusion. They found that before the earthquake (beginning from Oct.2001),the brightness temperature along the seismogenic fault became higher than that outside the fault,and affirmed that this phenomenon was the impending precursor of the 2001 Kunlunshan M_S 8.1 earthquake. However,our comparison study on the IR images of 2001 with those of 1999 has revealed that the same phenomenon has occurred also in 1999,in which no earthquake has been recorded. The Eastern Kunlun Fault is interpreted and analyzed by using NOAA thermal infrared (IR) remote sensing images combining with numerical processing of IR brightness temperature. The comparison of IR images in seismically quiet period of 1999 with those before and after the Kunlunshan M_S 8.1 earthquake in 2001 has indicated that seasonal meteorological factor greatly affects the change of IR images along seismogenic fault. In early winter,the IR brightness temperature along the fault is equal or even higher than that outside the fault. Moreover,the comparison between brightness temperature along the Eastern Kunlun Fault and along the Altyn Tagh Fault has also revealed that during the transitional period between autumn and winter,the disturbance of meteorological factors on ground surface are significantly greater than the IR of ground object itself. We suggest,therefore,that the anomalous increase of ground temperature before the occurrence of the M_S 8.1 Kunlunshan earthquake on Nov.14,2001 incorporated natural phenomenon of seasonal change,while the identification of anomaly related to earthquake needs a further study.